Text-to-speech (TTS) systems convert written text to sound. This can be useful to assist people with a visual impairment, dyslexia and other reading impairments, by reading text of computer screens and other electronic devices out loud. Speech recognition systems have also progressed to the point where humans can interact with computing devices entirely relying on speech. TTS and speech recognition combined with natural language understanding processing techniques enable speech-based user control and output of a computing device to perform tasks based on the user's spoken commands. The combination of speech recognition and natural language understanding processing techniques is commonly referred to as speech processing. Such TTS and speech processing may be used by computers, hand-held devices, telephone computer systems, kiosks, and a wide variety of other devices to improve human-computer interactions.